The ground loop issues of a USB cable can be mitigated by using an unpowered version (data only) of the USB cable. Several high end companies offer this sort of cable (Sablon Audio, for example).
Most standalone DAC's don't take power from the USB cable anyway. Power isn't the real culprit, the ground loop can occur in the data, not to mention extra noise. Ideally DAC's now have galvanic isolation built into all of the metal connectors.
If your DAC does take power from the USB cable, using an isolator with a linear power supply is a better solution.